After the Yeti invasion of the London Underground, the government realized that there was a need for an organization which could be set up to deal with new and unusual menaces to mankind, particularly those of the extraterrestrial variety. As a result, the United Nations Intelligence Taskforce, aka UNIT, was born.
Those in charge of UNIT made a policy decision not to tell the world about alien invasion attempts - when an invasion attempt occurs, UNIT feeds the press a cover story and cleans up the mess themselves. Technically, UNIT is a branch of the armed forces. During the 1980s, at least, UNIT is apparently headed by Sir John Sudbury of department C19.
During the third Doctor's exile on Earth, he joined UNIT in the capacity of an unpaid scientific adviser, and helped them foil numerous attempts to take over the Earth. Some of the UNIT members worked so closely with him that they practically achieved companion status, and these are listed below.
Brigadier Lethbridge-Stewart
Nicholas Courtney
Sergeant Benton
John Levene
Captain Yates
Richard Franklin
